The present invention relates to a fluid dynamic bearing device free from occurrence of leaks of lubricant and stable in floating characteristics or stable thrust floating characteristics against temperature changes of the working environment of the bearing device, and also to a motor to be used for disk recording devices that perform recording and reproduction of signals derived from a rotary magnetic disk equipped with the fluid dynamic bearing device.
FIG. 6 is a sectional view of a prior art example of the fluid dynamic bearing.
In this fluid dynamic bearing, a thrust plate 13 for closing a cylindrical hole is fixed at a lower end face of a sleeve 11, and a rotatable shaft 12 is inserted into the cylindrical hole.
An opening portion of the cylindrical hole is formed as a large-diameter hole 11a that is larger in diameter than the cylindrical hole.
On the outer circumferential surface of the shaft 12, a spiral-shaped dynamic pressure generating groove 12a is provided as shown in FIG. 7 in opposition to the inner circumferential surface of the cylindrical hole of the sleeve 11.
The dynamic pressure generating groove 12a performs an action of making the lubricant within a clearance between the shaft 12 and the sleeve 11 flow toward the thrust plate 13 during the operation of the bearing, i.e., during the rotation of at least one of the shaft 12 and the sleeve 11.
A circulation hole 12b is provided along the axial direction at a center portion of an end face of the shaft 12 confronting the thrust plate 13. Also, the shaft 12 is provided with a communication hole 12c that communicates with the large-diameter hole 11a from the circulation hole 12b during the operation of the shaft 12 (see, e.g., Japanese Unexamined Patent Publication No. 58-24616).
With the construction as shown above, the shaft 12, while at rest, has its one end face kept in contact with the thrust plate 13. When the shaft 12 is rotated, the lubricant within the large-diameter hole 11a is induced to flow toward the thrust plate 13 by a pumping action of the spiral-shaped dynamic pressure generating groove 12a, thereby making the shaft 12 float. As the shaft 12 is floated, the lubricant flows out through the circulation hole 12b and the communication hole 12c into the large-diameter hole 11a. Since the pressure between the thrust plate 13 and the one end face of the shaft 12 is adjusted by changes in the floating quantity of the shaft 12, stable floating characteristics can be obtained.
However, with such a construction as described above, there has been a serious drawback that when vibrations or shocks are applied in the axial direction, the lubricant flowing out to the large-diameter hole 11a provided at the opening of the sleeve 11 would be leaked outside the bearing, causing the lubricant to be exhausted.
FIG. 12 is a sectional view of a prior art example of a fluid dynamic bearing.
In this fluid dynamic bearing, a thrust plate 113 for closing a cylindrical hole 11g of a sleeve 111 is fixed at a lower end face of a sleeve holder 118 that surrounds the sleeve 111, and a rotatable shaft 112 is inserted into the cylindrical hole 111g. 
On the inner circumferential surface of the cylindrical hole 111g of the sleeve 111, asymmetrical herringbone-shaped dynamic pressure generating grooves 111b, 111c are provided as shown in FIG. 13 in opposition to the outer circumferential surface of the shaft 112.
Also, a spiral- or herringbone-shaped dynamic pressure generating groove 112b is provided on a thrust surface 112a confronting the thrust plate 113 of the lower end face of the shaft 112.
The dynamic pressure generating grooves 111b, 111c perform an action of making the lubricant within a clearance between the shaft 112 and the sleeve 111 flow toward the thrust plate 113 during the operation of the bearing, i.e., during the rotation of at least one of the shaft 112 and the sleeve 111.
With the construction as shown above, the shaft 112, while at rest, has its one end face kept in contact with the thrust plate 113. When the shaft 112 is rotated, an axially supporting force is generated by a pumping action of the dynamic pressure generating groove 112b of the thrust surface of the shaft 112, and moreover the lubricant within the clearance between the shaft 112 and the sleeve 111 is induced to flow toward the thrust plate 113 by a pumping action of the asymmetrical herringbone-shaped dynamic pressure generating grooves 111b, 111c of the inner circumferential surface of the sleeve 111, thereby making the shaft 112 float. Thus, under predetermined working environments, a specified thrust floating amount can be obtained (See, e.g. Japanese Unexamined Patent Publication No. 59-43216).
However, with such a construction as described above, there has been a serious drawback that the viscosity of the lubricant within the bearing would change due to temperature changes of the working environment (performance-guarantee temperature range for general bearing devices: −10 to 70° C.), so that the floating amount of the shaft would become lower than a specified value because of a decrease in the viscosity of the lubricant under high temperatures, while the floating amount of the shaft would become larger than a specified value because of an increase in the viscosity of the lubricant under low temperatures.